ENSURING SAFE AND NO-FAILURE OPERATION OF PIPELINES AND
POWER EQUIPMENT AFFECTED BY FLOW-ASSISTED CORROSION
Grigoriy V.Tomarov1 and Andrey A.Shipkov2
1
Professor, Moscow Power Engineering Institute, Moscow, Russia (geotherm@gmail.com) 2
Associate Professor, Moscow Power Engineering Institute, Moscow, Russia (ShipkovAA@mpei.ru)
ABSTRACT
NPP world service experience shows that the most commonly encountered pipelines damage mechanism is flow assisted corrosion, occurring in single- and two-phase flows [1-4]. Inner pipelines surfaces in condensate-feed and wet-steam parts of NPP secondary coolant circuit are subjected to general flow-assisted corrosion (GFAC). It influences on pipelines and power equipment reliability and service time and may reach the intensity 0.1 mm/year. GFAC is the main reason of pipelines walls general thinning and leads to coolant contamination with ferriferous species. As a result, operation life decreases and early power equipment breakdowns may occur.
Local flow-assisted corrosion (LFAC) mainly influences on NPP safe operation. It mainly occurs in pipeline parts of complex configuration and is characterized by high wear intensity up to 5.0 mm/year. LFAC leads to through-wall damages, pipelines and power equipment failures and severe breakdowns.
This paper describes novel system-wide approach, realized Russia to ensure pipelines and power equipment, affected by flow-assisted corrosion, safety and reliability. The efforts taken to fulfill the comprehensive program of measures to prevent pipelines and power equipment breakdowns and increase operational erosion-corrosion resistance in NPP secondary coolant circuit are reported.
NPP OPERATION ECONOMIC EFFICIENCY
Flow-assisted corrosion damages impair the NPP safety, reliability, operating efficiency and cause significant economic waste [5]. Generally, direct and consequential losses include electric energy underruns, operating expenditures and service maintenance cost (Fig. 1).
Fig. 1. Decrease of NPP operation economic efficiency due to flow-assisted corrosion
and perform diagnostics and repair works, steam generator and other equipment sediments cleaning, equipment preservation from atmospheric corrosion.
Significant economic losses are the result of emergency shut-downs, equipment failures, efficiency and NPP output decrease due to sediments formation, pressure drops and so on.
FAC CONSEQUENCES
GFAC and LFAC impacts in operating period are characterized by differential consequences and degree of safety and reliability reduction. The last-mentioned factors predetermine distinctions in measures to prevent pipeline and power equipment of NPP secondary coolant circuit local and general flow-assisted corrosion (Fig. 2).
Flow-assisted corrosion directly or indirectly affects adversely on metals service properties. On the first hand, LFAC leads to pipelines wall thinning, with subsequent NPP secondary coolant circuit depressurization. On the second hand, due to coolant contamination with GFAC ferriferous species, sediments formation in steam generator and other equipment takes place. Thirdly, FAC leads to metal damages and geometry of pipelines inner surfaces and roughness change, affecting on hydraulic friction. The first mentioned phenomenon influences upon NPP safety, the second and the third ones have an effect on reliability and efficiency of NPP operation.
Fig. 2. Pipelines and power equipment of NPP secondary coolant circuit local and general flow-assisted corrosion
Ensuring target life and overhaul-period renewal of NPP pipelines and equipment are mainly achieved by performing following procedures:
• in-service pipelines and power equipment metal inspection; • diagnostic and repair operation;
• water-chemistry maintenance.
In-service metal inspection, diagnostic and repair operation procedures are characterized by high cost, being the key aspects governing the prevention of FAC failures. Therefore their efficiency increase is the main factor to ensure pipelines and power equipment, affected by flow-assisted corrosion, safety and reliability.
METHODOLOGY OF FAC PROBLEM SOLVING
Solving the problem of ensuring target life and overhaul-period renewal of NPP pipelines and equipment is an urgent problem of contemporary nuclear power engineering that demands system-wide approach, based upon FAC physic-chemical processes and regularities scientific investigations, creation of computer modeling techniques to predict flow-assisted corrosion wear and, as a result, technical standard documentation working-out (fig. 3).
In recent years the numerical model of flow-assisted corrosion in single- and two-phase flows (RAMEK) has been developed in Russia [6, 7]. It is based upon physic-chemical criteria modeling of FAC processes and regularities and includes modules to estimate local FAC parameters and criteria taking into account technological processes occurring in condensate-feed and wet-steam parts of NPP secondary coolant circuit. RAMEK implements kinetic-migrational approach in LFAC modeling [8], that is determines local zones maximal LFAC intensity formation and their temporal development as well as LFAC zones migration alongside pipelines and NPP power equipment. RAMEK takes into account mutual influence of different factors, parameters and FAC-processes and being based on flow-assisted corrosion physics, ensures high adequacy of numerical modeling results. RAMEK implements such way of modeling that along with well-known influential factors calculates local values of mass-transport coefficient, hence defining local zones of maximum FAC-intensity.
Figure 3. Methodology of FAC problems solving
On the basis of RAMEK usage the concern “Rosenergoatom” developed and approved “The comprehensive program of measures to prevent pipelines and power equipment breakdowns and increase operational erosion-corrosion resistance in NPP secondary coolant circuit”.
The main aim of program implementation is to solve practical tasks of failures exclusion, breakdowns prevention, operational life prolongation and diagnostics and repair of pipelines and power equipment cost saving.
The main tasks of the Comprehensive program fulfillment are
• to provide scientific and technical support of control and diagnostic operations; • to improve normative technical documents;
• to work out and make use of NPP stuff support software systems to optimize planning and decision implementing, concerning pipelines and power equipment repair and overhaul-period renewal;
• to work out FAC damages databases and atlases of pipelines and power equipment LFAC charts in NPP secondary coolant circuit;
• to develop in-situ FAC monitoring.
Thus, the steam part of NPP secondary coolant circuit was nominally divided into I and II pipelines and power equipment groups in such a manner that the metal surface of pipelines and power equipment of group I is under contact with superheated steam, and the metal surface of group II is under contact with wet-steam. Active wall thinning in case of saturated steam flow (moisture content is less than 0,5%) in pipelines is practically excluded, since physic-chemical basis of such a process is absent. Meanwhile, stress corrosion cracking and other mechanisms of metal degradation, occurring without considerable wall thinning, may take place.
Liquid film formation all-over the metal surface of pipelines and power equipment belonging to group II, - regeneration system, high pressure preheater, low pressure preheater, moisture separator/reheater on the part of heating steam (when moisture content is more than 1,5-2,0%) provides conditions for flow-assisted corrosion and in specific cases cavitation erosion.
Virtually all the pipelines and power equipment elements of condensate part (group III) and feed part (group IV) of NPP secondary coolant circuit are subjected to flow-assisted corrosion. Elements of group III along with flow-assisted corrosion may experience cavitation erosion, so far as the value of pressure difference ∆Р=Р-Рsaturation is low, hence facilitating cavitation. And vice versa, elements of group IV may be subjected to cavitation erosion only in exceptional cases, so far as pressure difference is significant (operating pressure considerably exceeds saturation pressure).
Special attention must be applied to elements of group V, operating in corrosive aggressive medium – condensate (or separate) of heating steam.
Operating experience proves that a number of elements belonging both to wet-steam and condensate-feed parts of NPP secondary coolant circuit may experience atmospheric corrosion as predominant mechanism of wall thinning.
To reveal the dominating mechanism of metal thinning is the most important practical task so as the accuracy of maximum LFAC zone localization prediction and operating measures to prevent and eliminate FAC consequences optimization depend on it proper solution.
Mechanisms of metals damage discussed above have different origination, follow their own rules, differ in regularities and it is this fact must govern the measures to prevent failures and techniques to estimate pipelines and NPP power equipment residual life.
COMPREHENSIVE PROGRAM FULFILLMENT
Comprehensive program realization includes information analysis system creation with 3D-model of pipelines flow-channel development, results of numerical modeling, operation conditions, FAC-damages, elements metal chemistry databases and so on [9].
Fig. 5 displays a typical example of pressure part of feed-water pipelines identification card demonstrating geometry, position and elements distribution in groups depending on FAC remaining life. Such databases creation allows carrying out information analysis and applied problems solution to prevent FAC damages along with pipelines and power-equipment erosion-corrosion resistance increase.
Fig. 5. Pressure part of feed-water pipelines typical identification card and elements distribution in risk-group II (a) and risk-group III (b)
CONCLUSIONS
Comprehensive program realization enables to:
¾ improve NPP safety by way of accidents prevention due to pipelines and power equipment flow-assisted corrosion damages;
¾ reduce control and diagnosic opration expenditures;
¾ reduce diagnostic and repair expenditures eliminating flow assisted corrosion consequences and preventing FAC of pipelines and power-equipment (including steam generator and other NPP equipment sediments cleaning).
REFERENCES
1. Tomarov G.V. Erosion-corrosion of the Constructional Materials of Turbine Installations of Saturated Steam, J. Thermal Eng., 7, pp. 33-38, 1989.
2. Povarov O.A., Tomarov G.V., E.V. Velichko, et.al. Erosion-corrosion Wear of the Metal of the Elements of TPSs and NPSs (in Russian). A Survey, Energeticheskoe mashinostroenie, 3 (12), 1991.
3. Povarov O.A., Tomarov G.V. Erosion-corrosion of Energy Equipment Metals in Single and Two-phase Flows, J. Tyazheloye mashinostroeniye (in Russian), 8, 18-24, 2002
4. Flow-accelerated Corrosion in Power Plants. Chexal B., Horowitz J., Jones R. et al. // TR-106611. 504 p. 1996. 5. Akol’zin P.A. Corrosion and Protection of the Metal of the Thermal Power Equipment (in Russian), Moscow: Energoizdat, 304 p., 1982.
6. Tomarov G.V. The Physicochemical Processes and Regularities of Erosion–Corrosion of the Metal of the Power Equipment in Two-Phase Flow, J. Thermal Eng., 48(9), 761-770, 2001.
7. Tomarov G.V. and Shipkov A.A. Simulation of Physicochemical Processes of Erosion–Corrosion of Metals in Two-Phase Flows, J. Thermal Eng., 49(7), 530-541, 2002.
8. Tomarov G.V., Shipkov A.A. and Kasimovsky M.V. Kinetic-migrational approach to model pipelines and NPP power equipment local flow-assisted corrosion (in Russian). J. Energy efficiency and water treatment, 6(44), pp.8-12, 2006. 9. Tomarov G.V., Shipkov A.A., Semenov V.N. and Kasimovsky M.V. Pipelines and NPP power equipment metal control and diagnostic optimization (in Russian), J. Heavy Engineering, 1, pp.12-15, 2007.
